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Date:   Tue, 1 Oct 2019 16:43:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@ACULAB.COM>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH 13/15] x86/static_call: Add inline static call
 implementation for x86-64
Message-ID: <20191001144323.GR4519@hirez.programming.kicks-ass.net>
References: <20190605130753.327195108@infradead.org>
 <20190605131945.313688119@infradead.org>
 <20190610183357.zj6rwdpgw36anpfc@treble>
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On Mon, Jun 10, 2019 at 01:33:57PM -0500, Josh Poimboeuf wrote:
> On Wed, Jun 05, 2019 at 03:08:06PM +0200, Peter Zijlstra wrote:
> > --- a/arch/x86/include/asm/static_call.h
> > +++ b/arch/x86/include/asm/static_call.h
> > @@ -2,6 +2,20 @@
> >  #ifndef _ASM_STATIC_CALL_H
> >  #define _ASM_STATIC_CALL_H
> >  
> > +#include <asm/asm-offsets.h>
> > +
> > +#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
> > +
> > +/*
> > + * This trampoline is only used during boot / module init, so it's safe to use
> > + * the indirect branch without a retpoline.
> > + */
> > +#define __ARCH_STATIC_CALL_TRAMP_JMP(key, func)				\
> > +	ANNOTATE_RETPOLINE_SAFE						\
> > +	"jmpq *" __stringify(key) "+" __stringify(SC_KEY_func) "(%rip) \n"
> > +
> > +#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
> 
> I wonder if we can simplify this (and drop the indirect branch) by
> getting rid of the above cruft, and instead just use the out-of-line
> trampoline as the default for inline as well.
> 
> Then the inline case could fall back to the out-of-line implementation
> (by patching the trampoline's jmp dest) before static_call_initialized
> is set.

I think I've got that covered. I changed arch_static_call_transform() to
(always) first rewrite the trampoline and then the in-line site.

That way, when early/module crud comes in that still points at the
trampoline, it will jump to the right place.

I've not even compiled yet, but it ought to work ;-)